Human immunodeficiency virus type 1 (HIV-1) induces viral-to-cell membrane fusion to gain entry into target cells (8, 15, 66). The first high-affinity interaction between the virion and the cell surface is the binding of the viral surface glycoprotein gp120 to the CD4 antigen (13, 30, 41, 42). This in turn induces conformational changes in gp120, which enable it to interact with one of several chemokine receptors (4, 5, 21, 36). The CC-chemokine receptor CCR5 is the major co-receptor for macrophage-tropic (R5) strains, and plays a crucial role in the sexual transmission of HIV-1 (4, 5, 21, 36). T cell line-tropic (X4) viruses use CXCR4 to enter target cells, and usually, but not always, emerge late in disease progression or as a consequence of virus propagation in tissue culture (4, 5, 21, 36). Some primary HIV-1 isolates are dual-tropic (R5X4) since they can use both co-receptors, though not always with the same efficiency (11, 57). Mutagenesis studies coupled with the resolution of the gp120 core crystal structure demonstrated that the co-receptor-binding site on gp120 comprises several conserved residues (32, 53, 65).
It has been demonstrated that tyrosines and negatively charged residues in the amino-terminal domain (Nt) of CCR5 are essential for gp120 binding to the co-receptor, and for HIV-1 fusion and entry (6, 18, 20, 22, 28, 31, 52, 54). Residues in the extracellular loops (ECL) 1–3 of CCR5 were dispensable for co-receptor function, yet the CCR5 inter-domain configuration had to be maintained for optimal viral fusion and entry (24). This led to the conclusion either that gp120 forms interactions with a diffuse surface on the ECLs, or that the Nt is maintained in a functional conformation by bonds with residues in the ECLs. Studies with chimeric co-receptors and anti-CCR5 monoclonal antibodies have also shown the importance of the extracellular loops for viral entry (5, 54, 64).
Molecules that specifically bind to CCR5 and CXCR4 and block interactions with their ligands are a powerful tool to further probe the structure/function relationships of the co-receptors. Characterizing such compounds could also assist in designing effective therapeutic agents that target co-receptor-mediated steps of viral entry. Inhibitors of CCR5 or CXCR4 co-receptor function identified to date are diverse in nature and include small molecules, peptides, chemokines and their derivatives, and monoclonal antibodies (mAbs). The mechanisms of action of the small molecules that block entry by interfering with CXCR4 co-receptor function are not well understood (17, 49, 55, 68). One such inhibitor, the anionic small molecule AMD3100, depends on residues in ECL2 and the fourth trans-membrane (TM) domain of CXCR4 to inhibit viral entry, but it is not clear whether it does so by disrupting gp120 binding to CXCR4 or post-binding steps leading to membrane fusion (16, 34, 55) To date, no small molecules have been reported that specifically block CCR5-mediated HIV-1 entry. Inhibition of HIV-1 entry by chemokines is mediated by at least two distinct mechanisms: blockage of the gp120/co-receptor interaction and internalization of the chemokine/receptor complex (3, 26, 59, 63). The variant AOP-RANTES also inhibits recycling of CCR5 to the cell surface (40, 56). Variants such as RANTES 9–68 and Met-RANTES only prevent the gp120/CCR5 interaction and do not down-regulate CCR5 (67). SDF-1 variants presumably act through a similar mechanism to block viral entry mediated by CXCR4 (12, 27, 39). Only one anti-CXCR4 mAb, 12G5, has been characterized for its anti-viral properties. The efficiency of 12G5 inhibition of viral entry has been reported to be both cell- and isolate-dependent (43, 58). This mAb binds to the ECL2 of CXCR4, but the mechanism by which it inhibits entry is unknown (7). Few of the anti-CCR5 mAbs characterized to date efficiently prevent HIV-1 entry (28, 64). Interestingly, mAbs whose epitopes lie in the Nt domain of CCR5, which contains the gp120-binding site, inhibit viral fusion and entry less efficiently than mAb 2D7, whose epitope lies in ECL2. 2D7 also antagonizes CC-chemokine activity (64).
A panel of six murine mAbs, designated PA8, PA9, PA10, PA11, PA12 and PA14 have been isolated and characterized. All six mAbs specifically bound to CCR5+ cells but with different efficiencies that were cell type-dependent. Epitope mapping studies identified the residues that are important for mAb binding and also revealed information about the folding and interactions of the CCR5 extracellular domains. All mAbs inhibited HIV-1 fusion and entry, but there was no correlation between the ability of a mAb to inhibit fusion and entry and its ability to inhibit binding of gp120/sCD4 to CCR5+ cells.